Heat sink for plastic casings

ABSTRACT

A heat sink for mounting a semiconductor chip in a plastic casing. The upper surface of the heat sink is in thermal contact with the semiconductor chip, the lower surface is coplanar with a main surface of the casing. The heat sink is formed from a substantially square sheet of metal having each of its corners folded as a tongue to provide a substantially square base, each of the folded tongues forming the lower surface of the base.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor component casings, andmore particularly to multi-pin casings for integrated circuits formed bymolding a thermosetting plastic material.

2. Discussion of the Related Art

The power that can be dissipated by integrated circuits increases (withan increased density of circuits on silicon chips). Typically, the powerdissipation of integrated circuits has often been considered asnegligible. However, modern integrated circuits can now dissipate powerof the same order of magnitude as discrete components, for exampleapproximately 10 watts. A problem associated with this increased powerdissipation arises with the heat-conductive, thermosetting plasticcoating, such as a silicon epoxy resin, applied to the entire surface ofa silicon chip which can no longer ensure the required thermaldissipation.

Component casings, such as the one illustrated in the cross-sectionalview of FIG. 1, have been proposed to address this problem. This casing7, designed to provide protection for and connection of an integratedcircuit chip, includes a chip 1 disposed on a chip support portion 2 ofa lead frame that also includes connecting pins 3. Each connecting pin 3is connected to a pad of the chip 1 through a conductive wire 4.Conventionally, mounting the chip 1 to the support 2 is achieved with alayer of heat conductive adhesive 5. The chip 1 and support 2 assemblyare molded into a thermosetting resin in a manner that allows the pins 3to protrude externally from the casing 7. The casing 7 is often shapedas a square or rectangle and includes pins on each of the four sides.Typically, the casing is a few centimeters in sides and has a standard3.6-mm height. It is often referred to in the art as a "Plastic QuadFlat Fack" (PQFP). In addition, when the chip dissipates a relativelyhigh power, a heat sink base 8 is added, on which support 2 is placed,and which extends down to the lower main surface of the casing. The base8 may include, for example, fins 9 as shown in FIG. 1. This base 8 isgenerally made from a metal, usually copper or aluminum.

Conventionally, the base 8, which can have a 1 to 2-mm thickness, ispress-manufactured which, more particularly when made of aluminum,requires a complex and expensive manufacturing process. Another problemassociated with such a casing is that, after injection of resin into amold, upon hardening, its contraction ratio during the polymerizationstep is much higher than the thermal contraction of silicon and of thevarious metals, more particularly of the material of the base. As aresult, the casing 7 may be distorted and warped, and become concavenear its upper surface where the resin is thicker. This deformation canproduce various defects, including defective anchoring between the resinand the elements it contains, to such an extent that it can result indamage to the silicon chip.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a particularstructure for the heat sink base that solves both the manufacturing anddeformation problems.

This and other objects are achieved according to the invention with aheat sink for use in a plastic semiconductor casing that includes abase, an upper surface of the base being in thermal contact with asemiconductor chip, and a lower surface being coplanar with a mainsurface of the casing. The base is formed from a sheet of metal providedwith cuts, each of the cuts being folded under the area corresponding tothe upper surface of the base to form the lower surface.

According to an embodiment of the invention, the base is formed from asubstantially square sheet having each of its corners folded as a tongueto provide a substantially square base, each folded tongue forming thelower surface of the base.

According to an embodiment of the invention, the base includes aplatform, and a plurality of fins that are coupled thereto by connectingarms.

According to an embodiment of the invention, each connecting arm iscurved so that the lower surface of each fin is coplanar with the lowersurface of the platform.

According to an embodiment of the invention, the connecting arms of thefins substantially extend from the central portions of the platformsides before the tongues are folded.

According to an embodiment of the invention, the tongues are folded sothat they do not contact each other.

According to an embodiment of the invention, the folded tongues do notcontact each other at least near the corners of the substantially squarebase, so that the upper surface at least partially extends beyond thelower surface.

According to an embodiment of the invention, the sheet of metal, priorto folding, includes apertures near the fold lines, and at least on thetongues.

According to an embodiment of the invention, the base is formed from asheet of aluminum.

The foregoing and other objects, features, aspects and advantages of theinvention will become apparent from the following detailed descriptionof the present invention when taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1, described above, is a schematic cross-sectional view of aplastic casing with a heat sink;

FIG. 2A is a bottom view of a heat sink base according to an embodimentof the invention;

FIG. 2B is a cross-sectional view along line 2B--2B of FIG. 2A;

FIG. 2C is a cross-sectional view along line 2C--2C of FIG. 2A;

FIGS. 3A, 3B and 3C illustrate an alternative embodiment of the heatsink base according to the invention; and

FIGS. 4A, 4B and 4C illustrate another alternative embodiment of theheat sink base according to the invention.

DETAILED DESCRIPTION

As shown by the bottom view of FIG. 2A and the cross-sectional views ofFIGS. 2B and 2C, the heat sink 25 according to an embodiment of thisinvention includes a platform 10, fins 12 and connecting arms 13 thatconnect the fins 12 to the platform 10. The platform 10 is generallysquare shaped for receiving the chip support 2, shown in FIG. 1, on theupper surface 11 located opposite to the lower surface 27 shown in FIG.2A. An assembly of the platform 10, the chip support 2, and the chip 1is illustrated in FIG. 2C. The chip 1 may also be placed directly on theplatform 10.

The heat sink 25 is manufactured from a suitably cut sheet of metalhaving, for example, a 0.8-mm thickness. The center of the heat sink isinitially cut in the form of a square, with each corner of this squareconstituting a tongue 15 that is folded down onto the lower surface 27of the sheet, as shown in FIGS. 2A-2C. The pattern of a tongue 15 beforebeing folded is shown in phantom and designated as 16 in FIG. 2A. Asshown in FIGS. 2B and 2C, the connecting arms 13 between the platform 10and fins 12 are preferably curved so that the lower surface 29 of eachfin 12 is coplanar with the lower surface 31 of the folded tongues 15.Thus, after resin molding, the lower surfaces 29 of fins 12 are adjacentthe lower surface of the casing, and coplanar to the lower surfaces 31of tongues 15.

The fins 12 can be formed to any desired shape as chosen by thoseskilled in the art, but should preferably include apertures 18, such asthose shown in FIG. 2A, for ensuring a more satisfactory anchoring ofthe molding resin to the fins 12. The attainment of a good anchoring ofthe heat sink according to the invention in the resin is one of theproblems that the present invention aims at solving. In the embodimentof FIG. 2A, a particularly satisfactory anchoring is ensured due to thefact that the folded tongues 15 do not contact each other so that twoorthogonal channels 19 are created between them. During the moldingstep, some resin will flow into channels 19 and beneath the connectingarms 13, thus ensuring a satisfactory anchoring of the heat sink and amore even distribution of the resin between the volume existing near theupper side and the volume existing near the lower side of the casing.

According to an embodiment of the present invention illustrated in FIG.2C, the support portion 2 of the lead frame, which supports the chip 1,overhangs the platform 10 in order to further enhance the anchoring ofthe resin. Additionally, the size of the platform 10 is formed to belarger than that of the chip 1 to ensure sufficient thermal dissipationfrom the chip toward the lower surface 31 of the tongues 15.

FIGS. 3A, 3B and 3C, and FIGS. 4A, 4B and 4C respectively correspond toFIGS. 2A, 2B and 2C, which represent a top view and cross-sectionalviews along lines 2B--2B and 2C--2C of a base, to illustrate alternativeembodiments of the invention. In each of these drawings, same referencenumbers designate similar elements.

In an alternative embodiment as illustrated in FIGS. 3A-3C, prior tofolding each tongue 16, a window 21 is formed in the metallic sheetadjacent the folding line 33. When folded, an aperture 22 issubsequently formed in each tongue 15 as illustrated in FIGS. 3A and 3C.The apertures 22 allow the resin to penetrate beneath the platform 10thereby improving the anchoring of the heat sink 25. In an alternativeembodiment as illustrated in the lower right quarter of FIG. 3A, aradial aperture 22' is provided adjacent the fold line 33. In analternative embodiment as illustrated in the lower left quarter of FIG.3A, a radial aperture 22-1 is provided in the platform 11 with a largeraperture 22-2 provided in the corresponding tongue 15. The aperture canalso be formed in various other shapes such as a cross.

Additionally, it should be noted that during the folding process wherethe rough tongues 16 form tongues 15, no particular steps should betaken to ensure a smooth surface along the radius of the fold line 33.Should this radius crack during the folding process, it would ensurebetter anchoring to the resin.

As shown in the embodiment of FIGS. 4A-4C, the rough tongue patterns 16can be pre-cut to ensure that tongues 15 contact each other after theyare folded. This increases the thermal dissipation surface. However, inthis embodiment, it is preferred to precut the rough tongue pattern 16so that, after folding, recesses 23 are formed below the platform 10,extending from the connecting arms 13, to again ensure the properanchoring to the resin. The alternative embodiment of FIGS. 3A-3C can beadvantageously combined with that of FIG. 4A-4C.

One advantage of the structure according to the invention is that theplatform 10 has a two-sheet thickness that results in a substantiallyrigid surface that resists warping. This ensures a flat surface on whichto mount the chip support 2 and provide good thermal contact. The fins12, which can be formed with apertures 8, have a one-sheet thickness andtherefore form flexible structures that follow the contraction of theresin, thereby minimizing internal stresses.

As previously indicated above, the heat sink 25 according to theinvention readily lends itself to simple and inexpensive manufacturingprocesses since it can be fabricated by cutting and folding. It ispreferred, as is conventional in the field of industrial manufacturing,that a plurality of heat sinks 25 be sequentially formed from a ribbonor an elongated plate of material, where heat sinks remain integral withthe ribbon or plate until the final manufacturing steps. Thereafter,chemical, physical, thermal, electrothermal or physicochemical processescan be performed on the heat sinks while they are still grouped. If theheat sinks 25 are made of aluminum, anodic oxidation or any othertreatment for increasing the surface roughness of the heat sinks can beperformed, in order to enhance the anchoring thereof to the resin, toimprove their appearance, to get rid of polluting products, and so on.The tongues 15 can be folded simply by a punching process to provide, asdesired, a heat sink having its upper and lower surfaces plane andparallel one with respect to the other.

As it should be apparent to those skilled in the art, variousmodifications can be made to the above disclosed preferred embodiments,and more particularly for the shapes to be selected, namely the shape ofthe platforms 10, the shape of the folded tongues 15, and the shape andnumber of the fins 12. Similarly, although the implementation of thepresent invention is particularly advantageous when used with analuminum structure, it can also be embodied in heat sinks made of othermaterials, such as copper.

Having thus described one particular embodiment of the invention,various alterations, modifications, and improvements will readily occurto those skilled in the art. Such alterations, modifications, andimprovements are intended to be part of this disclosure, and areintended to be within the spirit and scope of the invention.Accordingly, the foregoing description is by way of example only and isnot intended to be limiting. The invention is limited only as defined inthe following claims and the equivalents thereto.

What is claimed is:
 1. A heat sink for use in a plastic semiconductorcasing, the heat sink being formed from a sheet of metal having aplurality of cut-out sections that define a plurality of tonguesextending from a central portion of the sheet, each tongue having atleast one edge established by one of the plurality of cut-out sections,each of the plurality of tongues being folded along a corresponding foldline so that its at least one edge overlaps the central portion to forma platform having an upper surface for being in thermal contact with asemiconductor chip, and a lower surface arranged to be coplanar with amain surface of the casing, at least two of the fold lines beingnonparallel relative to each other.
 2. The heat sink as recited in claim1, wherein the sheet of metal is square and has corners, each tonguebeing disposed at a corner of the sheet of metal and being folded alonga fold line to form a square platform.
 3. The heat sink as recited inclaim 2, wherein the tongues do not contact each other.
 4. The heat sinkas recited in claim 3, wherein the tongues do not contact each other atleast near the corners of the square platform so that at least a portionof the upper surface extends beyond the lower surface.
 5. The heat sinkas recited in claim 2, wherein the tongues have apertures near the foldlines.
 6. The heat sink as recited in claim 1, further comprising:aplurality of fins coupled to the platform; and a plurality of connectingarms coupling the fins to the platform.
 7. The heat sink as recited inclaim 6, wherein each fin has a lower surface and the connecting armsare curved so that the lower surface of each fin is coplanar with thelower surface of the platform.
 8. The heat sink as recited in claim 6,wherein the platform has a plurality of sides and the connecting armsextend from each side of the platform.
 9. The heat sink as recited inclaim 1, wherein the sheet of metal is a sheet of aluminum.
 10. A heatsink for mounting a semiconductor chip in a semiconductor casing, theheat sink comprising:a platform including an upper portion and a lowerportion, the upper portion having an upper surface for mounting a chipthereon, the lower portion having a recess therein for attaching theplatform to the semiconductor casing, the platform being comprised of asheet having a central portion and a plurality of tongues extending fromthe central portion, the upper portion including the central portion andthe lower portion including the plurality of tongues disposed to overlapand to be substantially parallel to the central portion; at least onefin coupled to the platform; and at least one connecting arm couplingthe at least one fin to the platform, the connecting arm having ends,one end of the arm being coupled to the platform and the other end beingcoupled to the fin.
 11. The heat sink as recited in claim 10, whereinthe sheet is square and has a plurality of corners, each of theplurality of tongues being disposed at a corner of the sheet.
 12. Theheat sink as recited in claim 10, wherein the recess in the lowerportion is a channel defined by adjacent tongues that are laterallyspaced from each other below the upper portion.
 13. The heat sink asrecited in claim 10, wherein the recess is a channel in the lowerportion.
 14. The heat sink as recited in claim 10, wherein the recess isdisposed below the at least one connecting arm.
 15. The heat sink asrecited in claim 10, wherein the lower portion has at least oneaperture.
 16. The heat sink as recited in claim 10, wherein the finincludes at least one aperture.
 17. The heat sink as recited in claim10, wherein the fin and the platform each has a lower surface, the lowersurface of the fin being substantially coplanar with the lower surfaceof the platform.
 18. The heat sink as recited in claim 17, wherein theconnecting arm is shaped so that the lower surface of the fin issubstantially coplanar with the lower surface of the platform.
 19. Theheat sink as recited in claim 18, wherein the connecting arm is curved.20. The heat sink as recited in claim 10, wherein the platform includesmeans for attaching the heat sink to the casing.
 21. The heat sink asrecited in claim 10, wherein the fin includes means for attaching theheat sink to the casing.
 22. A heat sink for mounting a semiconductorchip in a semiconductor casing, the heat sink comprising:a sheet ofmetal including a central portion and a plurality of tongues extendingfrom the central portion, each of the plurality of tongues beingconnected to the central portion at a corresponding one of a pluralityof fold lines and being folded along its corresponding fold line tooverlap the central portion, wherein at least two of the plurality offold lines are nonparallel relative to each other, and wherein thecentral portion and the folded tongues establish a platform having anupper surface adapted to mount the semiconductor chip thereon and alower surface to engage the semiconductor casing.
 23. The heat sink asrecited in claim 22, wherein the sheet of metal has a plurality ofcut-out sections establishing the central portion and the plurality oftongues.
 24. A semiconductor device comprising:a semiconductor chip; asemiconductor casing; and a heat sink including a platform having afirst surface on which the semiconductor chip is mounted and a secondsurface that engages the semiconductor casing, the platform being formedfrom a sheet of metal having a plurality of cut-out sections that definea plurality of tongues extending from a central portion of the sheet,each of the plurality of tongues having at least one edge established byone of the plurality of cut-out sections, each of the plurality oftongues being folded so that its at least one edge overlaps the centralportion to form the platform, at least one of the central portion andone of the plurality of tongues having an aperture extendingtherethrough, at least a portion of the aperture being disposed on thesecond surface with a portion of the semiconductor casing extendingtherethrough to anchor the platform to the casing.
 25. The semiconductordevice as recited in claim 24, wherein the semiconductor casing is madefrom a plastic material, the platform has a recess that receives theplastic material to anchor the platform to the casing, and the recess isdisposed below the central portion of the platform.
 26. Thesemiconductor device as recited in claim 25, wherein the heat sinkfurther includes a plurality of fins coupled to the platform.
 27. Thesemiconductor device as recited in claim 26, wherein at least one of theplurality of fins has an aperture through which the plastic materialextends to anchor the fin to the semiconductor casing.
 28. Asemiconductor device comprising:a semiconductor chip; a semiconductorcasing; and a heat sink including a platform comprising an upper portionand a lower portion, the upper portion having an upper surface on whichthe semiconductor chip is mounted, the lower portion having at least twointersecting recesses therein that receive a portion of the casing toattach the platform to the semiconductor casing, the platform beingcomprised of a sheet having a central portion and a plurality of tonguesextending from the central portion, the upper portion including thecentral portion and the lower portion including the plurality of tonguesdisposed to overlap and to be substantially parallel to the centralportion.
 29. The semiconductor device as recited in claim 28, whereinthe semiconductor casing is made from a plastic material, and the recessreceives the plastic material to anchor the platform to the casing. 30.The semiconductor device as recited in claim 29, wherein the heat sinkfurther includes a fin coupled to the platform.
 31. The semiconductordevice as recited in claim 30, wherein the fin has an aperture throughwhich plastic material extends to anchor the fin to the semiconductorcasing.
 32. A heat sink for mounting a semiconductor chip in asemiconductor casing, the heat sink comprising:a sheet of metalincluding a central portion and at least three individual tonguesindependently extending from the central portion, each of the tonguesbeing connected to the central portion at a fold line and being foldedalong its fold line to overlap the central portion so that the centralportion and the folded tongues establish a platform having an uppersurface for mounting a semiconductor chip thereon and a lower surface toengage a semiconductor casing.